Other things to check.

• Air filter. Better air flow to the engine improves mileage. This is a cheap replacement. You can also vacuum the exiting filter. I don't recommend vacuuming the air-filter more than a couple of times as fine particulates may not be removed. A good air filter costs about $25.00 and will repay you in two or three tank-fulls.
• Tire pressure. Add two to three pounds more air to your front tires for improved handling. Check air-pressure when tires are cold (after sitting overnight) for the most accurate reading. A good gauge is cheap ($5 to $10) and will pay for itself in a tank fill up or two.
• Change your spark-plugs. Get the auto parts people to gap the plugs for you and change them one at a time. Doing it yourself will cost you about 1/3 to 1/4 of going to a mechanic.
• Change your oil every 5,000 to 6,000 miles. Changing more often is an added expense that has no real monetary return...unless you are racing that car. A 3,000 mile oil change is a waste of money, oil, and your time.